You can port your existing number to a prepaid account, but that will close out your post-paid account and trigger a final invoice, including your tab balance. If the prepaid account is a new activation, you will be entitled to the $20 activation credit, even thoughyou are using your "old" number.

It also means starting all over with a new postpaid account with a new number and Ø tab if you wish to maintain two lines in service with no acquisition of a new handset.